Steering control apparatus, steering control system, and steering control program

ABSTRACT

Embodiments of the invention provide a computer program product embodied on a computer readable storage medium. The computer program product is encoded with instructions configured to control a controller to perform a process. The process includes setting a target steering angle for the vehicle based on a steering amount of the vehicle, setting a target torque difference between right and left steered wheels based on the target steering angle, and setting a target driving/braking force for the vehicle based on an acceleration amount and a braking amount of the vehicle. The process also includes setting each target torque for the right and left steered wheels based on the target torque difference and the target driving/braking force and controlling driving of the steered-wheel motors based on the target torque. The kingpin point is positioned closer to the vehicle on an inside portion of an inside sidewall of a steered wheel.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of U.S. patent application Ser. No.11/889,009, filed on Aug. 8, 2007, which claims the foreign prioritybenefit under 35 U.S.C. §119 of Japanese Patent Application No.2006-216412 filed on Aug. 9, 2006. The subject matter of the earlierfiled applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field

The present invention relates to a steering control apparatus, asteering control system and a steering control program for controlling asteering angle of a vehicle, particularly to those for controlling asteering angle of a vehicle in which a steer-by-wire is employed and atorque is separately provided for each steered wheel.

2. Description of Related Art

As disclosed in JPA H07-125643, there has been known a “steer-by-wire(SBW)” in which a mechanical connection between a steering wheel andsteered wheels of a vehicle is eliminated. In a vehicle employing theSBW technique, a sensor detects steering amount of a steering wheel,based on which steered wheels are steered by using a motor or motors, orby using a hydraulic mechanism.

In such a conventional technique, an actuator dedicated to steering(also referred to as a “steering actuator”) such as a motor or ahydraulic mechanism is required as a power source for steering thesteered wheels. Consequently, spare space for other components of thevehicle is restricted so that a steering-relevant design is constrained.

Therefore, in the light of such a disadvantage, it has been target toprovide a steering control apparatus, a steering control system and asteering control program in the SBW scheme, which eliminates a steeringactuator, thereby to enhance flexibility of the steering-relevantdesign.

SUMMARY

In accordance with an embodiment of the invention, there is provided acomputer program product embodied on a computer readable storage medium.The computer program product is encoded with instructions configured tocontrol a controller to perform a process for controlling the steeringof a steer-by-wire type vehicle. The vehicle includes steered-wheelmotors that apply separate torques to the right and left steered wheelsrespectively. A center of a contact face thereof and a kingpin pointthereof are offset in a lateral direction of the vehicle. The processincludes setting a target steering angle for the vehicle based on asteering amount of the vehicle, and setting a target torque differencebetween the right and left steered wheels based on the target steeringangle. The process also includes setting a target driving/braking forcefor the vehicle based on an acceleration amount and a braking amount ofthe vehicle, and setting each target torque for the right and leftsteered wheels based on the target torque difference and the targetdriving/braking force. Further, the process includes controlling drivingof the steered-wheel motors based on the target torque. The kingpinpoint is positioned closer to the vehicle on an inside portion of aninside sidewall of a steered wheel.

In accordance with an embodiment of the invention, there is provided asteering control system that controls steering of a steer-by-wire typevehicle including steered-wheel motors that apply separate torques tothe right and left steered wheels respectively. A center of a contactface thereof and a kingpin point thereof are offset in a lateraldirection of the vehicle. The system includes a target steering anglesetting unit configured to set a target steering angle for the vehiclebased on a steering amount of the vehicle, and a target torquedifference setting unit configured to set a target torque differencebetween the right and left steered wheels based on the target steeringangle. The system further includes a target driving/braking forcesetting unit configured to set a target driving/braking force for thevehicle based on an acceleration amount and a braking amount of thevehicle, and a target torque setting unit configured to set each targettorque for the right and left steered wheels based on the target torquedifference and the target driving/braking force. Further, the systemincludes a motor driving unit for controlling driving of thesteered-wheel motors based on the target torque. The kingpin point ispositioned closer to the vehicle on an inside portion of an insidesidewall of a steered wheel.

Other features and advantages of the present invention will become moreapparent from the following detailed description of the invention whentaken in conjunction with the accompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments, details, advantages, and modifications of theinvention will become apparent from the following detailed descriptionof the preferred embodiments, which is to be taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a vehicle for a steering controlapparatus, in accordance with an embodiment of the invention, isprovided.

FIG. 2 is a view for explaining a structure in a vicinity of steeredwheels of the vehicle for which the steering controller, in accordancewith an embodiment of the invention, is provided, in which the steeredwheels are seen upward from a contact point side thereof.

FIG. 3 is a view on an arrow X, which is seen in a direction from a rearside toward a front side of the vehicle, in accordance with anembodiment of the invention.

FIGS. 4A to 4C are views for explaining steering of the steered wheelsby utilizing a kingpin offset, in accordance with an embodiment of theinvention.

FIGS. 5A and 5B are further views for explaining steering of the steeredwheels by utilizing a kingpin offset of zero and non-zero, respectively,in accordance with an embodiment of the invention.

FIG. 6 is a block diagram illustrating a steering controller, inaccordance with an embodiment of the invention.

FIG. 7 is a schematic diagram illustrating a vehicle for which asteering control apparatus, in accordance with an embodiment of theinvention, is provided.

FIG. 8 is a schematic diagram for explaining a lock mechanism, inaccordance with an embodiment of the invention.

FIG. 9 is a block diagram illustrating a steering controller, inaccordance with an embodiment of the invention.

FIG. 10 is a block diagram illustrating a steering controller, inaccordance with an embodiment of the invention.

FIG. 11 is a lock coefficient map for selecting a lock coefficient, inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION

The principle of the present invention is that a center of the contactface and a kingpin point are intentionally offset in a lateral direction(width direction) of a vehicle, and the vehicle is controlled in itssteering angle by utilizing this kingpin offset.

With reference to attached drawings, there will be provided descriptionson embodiments of the present invention, hereinafter. Same numeralreferences will be given to same or similar components, and duplicateddescriptions will be omitted. Right-left symmetric components will beprovided with numeral references of “R” and “L” respectively, ifnecessary.

FIG. 1 is a schematic diagram of a vehicle for a steering controlapparatus, in accordance with an embodiment of the invention, isprovided. FIG. 2 is a view for explaining a structure in a vicinity ofsteered wheels of the vehicle for which the steering controller, inaccordance with an embodiment of the invention, is provided, in whichthe steered wheels are seen upward from a contact point side thereof.

FIG. 3 is a view on an arrow X, which is seen in a direction from a rearside toward a front side of the vehicle, in accordance with anembodiment of the invention. FIGS. 4A to 4C are views for explainingsteering of the steered wheels by utilizing a kingpin offset, inaccordance with an embodiment of the invention. FIGS. 5A and 5B arefurther views for explaining steering of the steered wheels by utilizinga kingpin offset of zero and non-zero, respectively, in accordance withan embodiment of the invention. FIG. 6 is a block diagram illustrating asteering controller, in accordance with an embodiment of the invention.It should be noted that FIGS. 4 and 5 are schematic diagrams in whichthe kingpin offset is emphasized and tie rods 16 a, 16 b are simplified.

As shown in FIG. 1, the steering control apparatus, in accordance withan embodiment of the invention, is an apparatus for controlling steeringof the vehicle CR of steer-by-wire type, and includes motors M_(R) andM_(L) that provide separate torques for right and left steered wheelsrespectively; a rod member 16 that couples the right and left steeredwheels T_(R), T_(L) so that the right and left steered wheels T_(R),T_(L) can be steered; and a steering controller (steering controlsystem) 20A.

The steering controller 20A serves for controlling each torque for theright and left steered wheels T_(R), T_(L) of the vehicle CR, so as torealize a target driving/braking force and a target steering angle.

The steering controller 20A is connected with a steering sensor 31, anacceleration stroke sensor 32, a braking effort sensor 33 and a steeringangle sensor 34.

The steering sensor 31 serves for detecting steering amount of asteering ST (also referred to as merely “steering amount”) and asteering direction of the steering ST (also referred to as merely a“steering direction”). The acceleration stroke sensor 32 serves fordetecting acceleration amount of an accelerator pedal AP (also referredto as merely “acceleration amount”). The braking effort sensor 33 servesfor detecting braking amount of a braking pedal BP (also referred to asmerely “braking amount”). The steering angle sensor 34 serves fordetecting a steering angle α_(CR) of the vehicle CR.

Each result from the sensors 31 to 34 is output to the steeringcontroller 20A.

The steering controller 20A includes, for example, CPU, RAM, ROM and I/Ocircuits and executes various controls (described later) by executingcalculations based on each input from the sensors 31 to 34 and programsand data stored on the ROM.

As shown in FIGS. 2 and 3, the steer-by wire technique is employed inthe vehicle CR for which the steering control apparatus of the presentinvention is provided, and includes the motors M_(R), M_(L) respectivelyprovided for the right and left steered wheels T_(R), T_(L). The motor M(M_(R), M_(L)) is installed in the corresponding steered wheel T (T_(R),T_(L)), so as to provide a separate torque for each steered wheel T(in-wheel-motor system). The motors M_(R), M_(L) are each exemplified asa steered-wheel motor.

The vehicle CR employs a strut suspension scheme, and each steered wheelT is provided with a stabilizer 12 via a lower arm 11, a shock absorberunit 13 and a radial rod 14.

The rod member 16 coupling the right and left steered whleels T_(R),T_(L) includes a central rod 16 a and tie rods 16 b, 16 b that couplethe central rod 16 a with the steered wheels T_(R), T_(L) respectively.

Rn denotes a lower arm length (described later) which is a distance fromeach joint between the steered wheels T_(R), T_(L) and the respectivetie rods 16 b, 16 b to the corresponding kingpin point P2 (describedlater) in the longitudinal direction of the vehicle CR.

In the vehicle CR in which such a strut suspension system is employed, avirtual kingpin axis L is defined by a straight line through a joint(upper mount portion) A1 at which the shock absorber unit 13 and avehicle body of the vehicle CR are jointed and a joint (lower mountportion) A2 at which the motor M and the lower arm 11 are jointed.

The virtual kingpin axis L serves as a turning center axis when eachsteered wheel T turns.

A distance between a ground contact of a center line of the steeredwheel T (i.e. center of the contact face of the steered wheel T) P1 anda ground contact of the virtual kingpin axis L (i.e. kingpin point) P2in the lateral direction of the vehicle CR is referred to as a kingpinoffset (or scrub radius) Ro in the descriptions of the presentinvention.

As shown in FIG. 5A, if the kingpin offset Ro is zero, the steeredwheels T_(R), T_(L) turn about the center of the contact face P1 (=P2)thereof as a turning axis when being steered (only the right steeredwheel T_(R) is shown in FIG. 5A). Meanwhile, as shown in FIG. 5B, if thekingpin offset Ro is not zero, the steered wheels T_(R), T_(L) turnabout the kingpin point P2 thereof as a turning axis when being steered,and the center of the contact face P1 moves (turns) in a circular arcwith a radius of Ro (only the right steered wheel T_(R) is shown in FIG.5B). The present invention utilizes this turning movement of the centerof the contact face P1 about the kingpin point P2 for steering thevehicle CR.

A conventional vehicle is designed to reduce this kingpin offset Ro asmuch as possible. To the contrary, the present invention encourages thiskingpin offset Ro which is utilized in steering of the vehicle CR.

In this embodiment, as shown in FIG. 3, the steered-wheel motors M_(R),M_(L) are provided for the right and left steered wheels T_(R), T_(L)respectively. A distance (depth) WM of each motor M_(R), M_(L) of thesteered wheels T_(R), T_(L) in the lateral direction of the vehicle CRis set to be smaller than a distance WT of each steered wheel T_(R),T_(L) on the right and left side in the lateral direction of the vehicleCR (only the components on the left side are shown in FIG. 3). Thesteered-wheel motors M_(R), M_(L) are completely housed in the steeredwheels T_(R), T_(L), respectively, thereby to enhance flexibility ofsuspension geometry including the lower arm 11 and the shock absorberunit 13, etc., and to facilitate setting optimum offset variable of thekingpin point P2.

In particular in this embodiment, as shown in FIG. 3, each joint (lowermount portion A2) between the rod member 16 and the right and leftsteered wheels T_(R), T_(L) is provided more inward than each inner endL_(IN) of the right and left steered wheels T_(R), T_(L) in the lateral(right and left) direction of the vehicle CR. Accordingly, it ispossible to realize a greater kingpin offset than that of a conventionalvehicle.

In addition, the shock absorber 13 and the lower arm 11 and the like aredisposed such that the kingpin P2 is positioned closer to the vehicle CRside than from the inner end L_(IN), so that an efficient steeringeffort can be generated.

In the present invention, the bigger the kingpin offset Ro becomes, theeasier it becomes to generate a steering effort Fstr. Now, it should benoted that, if the kingpin offset Ro is set to be too great, movement ofthe tire Ty becomes too great when it is steered, which causes adisadvantage that the vehicle CR needs more housing space (tire house)for the tire Ty. In the light of this disadvantage, the kingpin offsetRo should be appropriately set in consideration of a body size, a tireradius Rt, a lower arm length Rn, and suspension geometry, etc., of thevehicle CR.

When the vehicle CR runs, the torques T_(—R), T_(—L) of the right andleft steered wheels T_(R), T_(L) each serve as a reaction force betweenthe road and each tire Ty, so as to generate driving forces Fd_(R),Fd_(L) (only the right steered wheel T_(R) is shown in FIG. 4A). Thedriving forces Fd_(R), Fd_(L) are represented by the following formulas1 and 2.

Fd _(L) =T _(—L) /Rt  [Formula 1]

Fd _(R) =T _(—R) /Rt  [Formula 2]

Where, Rt denotes a tire radius (radius of the tire Ty of the steeredwheel T).

The driving forces Fd_(R), Fd_(L) work on each tire ground contactcenter P1, and generate moments M_(R), M_(L) that steer the steeredwheels T_(R), T_(L) around the kingpin point P2, respectively (only theright steered wheel T_(R) is shown in FIG. 4B). The moments M_(R), M_(L)are represented by the following formulas 3 and 4.

M _(L) =Fd _(L) ·Ro  [Formula 3]

M _(R) =Fd _(R) ·Ro  [Formula 4]

Where the right and left steered wheels T_(R), T_(L) have both an equalkingpin offset Ro.

The tie rods 16 b, 16 b are approximately vertical to the respectivesteered wheels T_(R), T_(L), and forces along the axis of the tie rods16 b, 16 b (the rod member 16) caused by the respective moments M_(R),M_(L) denote as steering efforts Fstr_(—R), Fstr_(—L) respectively; thisrelation is represented by the following formulas 5 and 6 (in FIG. 4C,an upper view shows the steering effort Fstr_(—R) on the right steeredwheel in details, and a lower view shows both the steering effortsFstr_(—R) and Fstr_(—L) on the right and left steered wheels). It shouldbe noted that each arrow of the steering efforts Fstr of FIG. 4Crepresents a forward direction thereof, for convenience.

M _(L) =Fstr _(—L) ·Rn  [Formula 5]

M _(R) =Fstr _(—R) ·Rn  [Formula 6]

Where, Rn denotes the lower arm length, which is a distance from a jointbetween each steered wheel T and the tie rod 16 b to each kingpin pointP2 in the longitudinal direction of the vehicle CR.

Through these formulas 1 to 6, a total steering effort Fstr acting onthe rod member 16 is a force in a direction represented by a bold arrowFstr of FIG. 2, and is expressed by the following formulas 7 and 8.

Fstr=ΔT×Ro/(Rn×Rt)  [Formula 7]

ΔT=T _(—R) −T _(—L)  [Formula 8]

Where, Ro denotes the kingpin offset, Rn denotes the lower arm length,Rt denotes a tire radius (radius of the tire Ty of the steered wheel T).

ΔT is a difference between the torque T_(—R) of the right steered wheelT_(R) and the torque T_(—L) of the left steered wheel T_(L). Herein, itis defined that the right direction of FIG. 3 becomes a forwarddirection of Fstr.

Specifically, if the driving/braking forces (i.e. torques) of the rightand left steered wheels T_(R), T_(L) are both equivalent to each other,the steering efforts of the right and left steered wheels T_(R), T_(L)derived from the kingpin offset Ro are canceled by each other, so thatthere is generated no steering effort to the vehicle CR.

On the other hand, if the driving/braking forces (i.e. torques) of theright and left steered wheels T_(R), T_(L) are not equivalent to eachother, there are generated a difference between steering efforts of theright and left steered wheels T_(R), T_(L). Accordingly, there isgenerated any steering effort to the vehicle CR.

Even a vehicle (steering control system) employing a suspension systemother than such a strut suspension system also has a virtual kingpinaxis. Therefore, a kingpin offset may be actively applied to such avehicle, similar to this embodiment of the present invention.

As shown in FIG. 6, the steering controller (also referred to as a“steering control system”) 20A includes a target steering angle settingunit 21, a target torque difference setting unit 22, a targetdriving/braking force setting unit 23 and a target torque setting unit24 and a motor driving unit 25, as various functional units.

The target steering angle setting unit 21 acquires steering amount and asteering direction detected by the steering sensor 31, and set a targetsteering angle αm based on the steering amount and the steeringdirection that have been acquired.

The target steering angle αm is a target value for a steering angle ofthe vehicle CR. This target steering angle αm may be set to have thesame steering feeling of the steering actuator constituted of aconventional rack & pinion mechanism, hydraulic mechanism or the like.The target steering angle αm may also be set such that gain thereof isadjusted depending on the vehicle speed.

The target steering angle αm that has been set is output to the targettorque difference setting unit 22.

The target torque difference setting unit 22 sets a target torquedifference ΔTm based on the target steering angle αm and an actualsteering angle α_(CR) (see FIG. 1) detected by the steering angle sensor34 such that the actual steering angle α_(CR) agrees with the targetsteering angle αm. In this embodiment, the target torque differencesetting unit 22 calculates the target torque difference ΔTm by a PIcontrol using a difference Δα between the actual steering angle α_(CR)and the target steering angle αm (=α_(CR)−αm).

A relation between the target torque difference ΔTm and the differenceΔα is represented by the following formula 9.

ΔTm=kp·Δα+ki∫Δαdt  [Formula 9]

Where, kp denotes a proportional gain and ki denotes an integral gain,and these parameters are predefined based on results from a pretest orthe like.

The target torque difference ΔTm represents a difference between targettorques for the right and left steered wheels T_(R), T_(L) of thevehicle CR. This target torque difference ΔTm is represented by thefollowing formula 10.

ΔTm=Tm _(—) R−Tm  [Formula 10]

The target torque difference ΔTm that has been set is output to thetarget torque setting unit 24.

The target torque difference ΔTm may be set by not only the PI controlscheme but also other control schemes.

The target driving/braking force setting unit 23 sets a targetdriving/braking force Tm_total, based on the acceleration amountdetected by the acceleration stroke sensor 32 and the braking amountdetected by the braking effort sensor 33.

The target driving/braking force Tm_total is a target value for thedriving/braking force (torque) of the vehicle RC.

The target driving/braking force Tm_total that has been set is output tothe target torque setting unit 24.

The target torque setting unit 24 sets target torques Tm_R, Tm_L of theright and left steered wheels T_(R), T_(L).

The target torques Tm_R, Tm_L are a target value for each torque appliedto the right and left steered wheels T_(R), T_(L). Where, with thetarget torque Tm_R for the right steered wheel T_(R) and the targettorque Tm_L for the left steered wheel T_(L), the target torquedifference ΔTm and the target driving/braking force Tm_total arerepresented by the following formulas 11 and 12 respectively.

ΔTm=Tm _(—) R−Tm _(—) L  [Formula 11]

Tm_total=Tm _(—) R+Tm _(—) L  [Formula 12]

Therefore, the target torques Tm_R, Tm_L are represented by thefollowing formulas 13 and 14 respectively.

Tm _(—) L=(Tm_total−ΔTm)/2  [Formula 13]

Tm _(—) R=(Tm_total+ΔTm)/2  [Formula 14]

The target torques Tm_R, Tm_L that have been set are input to the motordriving unit 25.

The motor driving unit 25 controls driving of the motors M_(R) and M_(L)based on the target torques Tm_R, Tm_L. The motor driving unit 25provides a feedback control for the motors M_(R), M_(L) respectivelysuch that a torque of the motor M_(R) agrees with the target torque Tm_Rand such that a torque of the motor M_(L) agrees with the target torqueTm_L. In this embodiment, as the motors M_(R), M_(L), a three-phasebrushless motor is employed, and the motor driving unit 25 performs avector control based on a dq transformation. Accordingly, the drivingforce Tm_total and the steering angle α_(CR) that a driver of thevehicle CR desires are obtained at a time. In other words, a steeringeffort Fstr represented by the formula 7 is generated in response to anoutput command of generating ΔTm.

According to the steering control apparatus, in accordance with anembodiment of the invention, the vehicle CR can be controlled insteering without using a steering actuator. By adjusting the torquesapplied to the right and left steered wheels T_(R), T_(L), a targetsteering angle α_(CR) can be obtained. Accordingly, a steering actuatorcan be eliminated, thereby to enhance flexibility of steering-relevantdesign. In addition, since the steer-by-wire is employed, no mechanicalconnection between the steering ST and the steered wheels T_(R), T_(L)is required, thereby to highly enhance flexibility of vehicle design.

There will be provided descriptions on a vehicle in which a steeringcontrol apparatus, in accordance with an embodiment of the invention, isincorporated, mainly on different features from those of an embodimentof the invention. Descriptions on the same or similar features andcomponents of the previously described embodiment will be omitted.

FIG. 7 is a schematic diagram illustrating a vehicle for which asteering control apparatus, in accordance with an embodiment of theinvention, is provided.

FIG. 8 is a schematic diagram for explaining a lock mechanism, inaccordance with an embodiment of the invention. FIG. 9 is a blockdiagram illustrating a steering controller, in accordance with anembodiment of the invention.

As shown in FIG. 7, the steering control apparatus, in accordance withan embodiment of the invention, further includes a steering controller(also referred to as a “steering control system”) 20B, instead of thesteering controller 20A in the previously described embodiment.

The steering control apparatus, in accordance with an embodiment of theinvention, further includes the lock mechanism 17.

The locked mechanism 17 serves for locking the rod member 16 so as tofix the steering angle α_(CR), and is controlled in its driving by thesteering controller 20B.

As shown in FIG. 8, the lock mechanism 17 is a hydraulic cylindermechanism in which a rod 16 a serves as a piston rod, including acylinder 17 a divided into two partitions by a partition wall 16 c ofthe rod 16 a, a hydraulic passage 17 b that connects the two separatedpartitions and an electromagnetic valve 17 c provided in the hydraulicpassage 17 b.

The cylinder 17 a and the hydraulic passage 17 b are filled with oil,and the cylinder 17 a is fixed to the vehicle body. The electromagneticvalve 17 c is switchable between an open state and a close statethereof.

In the open state of the electromagnetic valve 17 c, the oil flowsbetween the two partitions of the cylinder 71 a, so that the rod 16 acan move laterally (in the right and left direction) and the steeredwheels T_(R), T_(L) can be steered.

On the other hand, in the close state of the electromagnetic valve 17 c,the oil cannot flow between the two partitions of the cylinder 71 a, sothat the steered wheels T_(R), T_(L) are locked.

Such a control by the electromagnetic valve 17 c is executed by a lockmechanism driving unit 27 (described later).

As shown in FIG. 9, the steering controller (steering control system)20B, in accordance with an embodiment of the invention, further includesa lock necessity determining unit 26 and the lock mechanism driving unit27, as the various function units.

In accordance with an embodiment of the invention, the target steeringangle αm that has been set by the target steering angle setting unit 21is also output to the lock necessity determining unit 26, and thesteering angle α_(CR) of the vehicle CR that has been detected by thesteering angle sensor 34 is output to the lock necessity determiningunit 26, as well.

The lock necessity determining unit 26 determines whether or not the rodmember 16 is required to be locked by the lock mechanism 17 based on adifference between the actual steering angle α_(CR) of the vehicle CRand the target steering angle αm. The lock necessity determining unit 26determines that the rod member 16 is required to be locked if thedifference Δα between the actual steering angle α_(CR) of the vehicle CRand the target steering angle αm (=α_(CR)−αm) is equal to apredetermined (threshold) value αTH or smaller.

This predetermined value αTH (also referred to as a “first predeterminedvalue”) is predetermined and presorted, and may be a constant value or avariable value varying in correlation with the vehicle speed.

A determined result from the lock necessity determining unit 26 isoutput to the lock mechanism driving unit 27.

The lock mechanism driving unit 27 controls driving of the lockmechanism 17 based on a determined result. If determining that the rodmember 16 is required to be locked, the lock mechanism driving unit 27controls the lock mechanism 17 to lock the rod 16 a. If determining thatthe rod member 16 is not required to be locked, the lock mechanismdriving unit 27 controls the lock mechanism 17 to unlock the rod 16 a.

In the steering control apparatus, in accordance with an embodiment ofthe invention, if there is no necessity of changing the steering angleα_(CR) when the vehicle runs straight, for example, stability of thesteered wheels T_(R), T_(L) can be enhanced.

In addition, in accordance with an embodiment of the invention, in acase in which a relatively greater turning radius is required for thesteered wheels T_(R), T_(L), a traveling direction of the vehicle bodycan be changed by utilizing a yaw moment due to a torque difference ΔTbetween the right and left steered wheels T_(R), T_(L) even in thelocked state of the rod member 16.

There will be provided descriptions on a vehicle in which a steeringcontrol apparatus, in accordance with an embodiment of the invention, isincorporated, mainly on different features from those of anotherembodiment of the invention. Descriptions on the same or similarfeatures and components of the previously described embodiments will beomitted.

FIG. 10 is a block diagram illustrating a steering controller, inaccordance with an embodiment of the invention. FIG. 11 is a lockcoefficient map for selecting a lock coefficient, in accordance with anembodiment of the invention.

The steering control apparatus, in accordance with an embodiment of theinvention, includes a lock mechanism 18 instead of the lock mechanism 17of another embodiment of the invention.

The lock mechanism 18 is designed to continuously or intermittentlyprovide a locking force to lock the rod 16 a.

The lock mechanism 18 can be realized by replacing the electromagneticvalve 17 c of the lock mechanism 17 of another embodiment of theinvention with a liner solenoid valve. Therefore, descriptions on thelock mechanism 18 will be omitted.

As shown in FIG. 10, the steering controller (also referred to as a“steering control system”) 20C, in accordance with an embodiment of theinvention, includes a lock coefficient setting unit 28 instead of thelock necessity determining unit 26. The steering controller 20C furtherincludes a vehicle speed calculating unit 29.

In this embodiment, the target steering angle αm that has been set bythe target steering angle setting unit 21 is output to the lockcoefficient setting unit 28, and the steering angle α_(CR) of thevehicle CR that has been detected by the steering angle sensor 34 isalso output to the lock coefficient setting unit 28.

The vehicle speed calculating unit 29 calculates a vehicle speed V_(CR)based on the torques of the motors M_(R), M_(L). The torques of themotors M_(R), M_(L) have a correlation with the wheel speed of thesteered wheels T_(R), T_(L), respectively. Calculating the vehicle speedV_(CR) based on speed of the driving wheels is a conventional method,and the vehicle speed calculating unit 29 may calculate the vehiclespeed V_(CR) by using such a conventional method.

The calculated vehicle speed V_(CR) is output to the lock coefficientsetting unit 28.

The lock coefficient setting unit 28 sets a lock coefficient K withreference to a lock coefficient map for selecting a lock coefficient,based on the vehicle speed V_(CR), an actual steering angle α_(CR) and atarget steering angle αm.

The lock coefficient K that has been set is output to the lock mechanismdriving unit 27.

The lock mechanism driving unit 27 controls driving of the lockmechanism 18 such that the lock coefficient K that has been set isachieved.

Now, more detailed descriptions will be given on the lock coefficient K.The lock coefficient setting unit 28 previously stores the lockcoefficient map for selecting a lock coefficient of the FIG. 11, whichrepresents a correlation of the difference Δα between the actualsteering angle α_(CR) of the vehicle CR and the target steering angle αm(=α_(CR)−αm), the vehicle speed V_(CR) and the lock coefficient K, asshown in FIG. 11.

The lock coefficient K is a value between 0 and 1, in which “K=0”denotes a completely unlocked state (a state in which a lock iscompletely released), and “K=1” denotes a completely locked state.

If such a lock coefficient K is used, the steering effort Fstr acting onthe rod member 16 is represented by the following formula 15.

Fstr=(1−K)×ΔT×Ro/(Rn×Rt)  [Formula 15]

Specifically, by setting an appropriate lock coefficient K, it ispossible to continuously or intermittently control the relation betweenthe torque difference ΔT and the steering effort Fstr.

In accordance with an embodiment of the invention, the lock coefficientK is set such that the greater the difference Δa becomes, the smallerthe lock coefficient K becomes; and the greater the vehicle speed V_(CR)becomes, the greater the lock coefficient K becomes. In addition, whenthe difference Δα is equal to a predetermined (threshold) value ΔαTH(also referred to as a “second predetermined value”) or greater, thelock coefficient K is set to become constant for each vehicle speed VC.

Therefore, in a lower speed driving (“vehicle speed V_(CR) small” ofFIG. 11), the vehicle body orientation can be changed with a smallersteering radius by a steering; and in a higher speed driving (“vehiclespeed V_(CR) great” of FIG. 11), the change of the vehicle bodyorientation by the steering can be restrained, so as to secure stabilityof the vehicle body, meanwhile the vehicle body orientation can bechanged mainly by using a yaw moment due to the torque difference ΔTbetween the right and left steered wheels T_(R), T_(L).

The lock coefficient map for selecting the lock coefficient K of FIG. 11may be set appropriately, depending on output performance of the motorM, an operation feeling of the steering ST, and movement characteristicsdetermined by a structure of the vehicle body CR, etc. If asignificantly greater yaw moment can be set because of an outputperformance of the motor M, or if a stability of the vehicle body isattached more importance to, the lock coefficient map of FIG. 11 may beset such that each line has a more moderate inclination within a rangeof the difference Δα between 0 to ΔαTH; or ΔαTH (i.e. the bend point ofthe lines in FIG. 11) is set within a relatively smaller range of thedifference Δα.

According to the steering control apparatus, in accordance with anembodiment of the invention, it is possible to change stability andsteering response depending on the condition of the vehicle CR.

“Steering response” means a response performance from a time when thesteering ST is manipulated by a driver to a time when the vehiclechanges its orientation, and also means a ratio between steering amount(input steering angle) of the steering ST and orientation variable ofthe vehicle body.

The embodiments of the present inventions have been explained in detailsas described above. However the present invention is not limited tothose specific embodiments, and various modifications and changesthereto can be made by persons skilled in the art without departing fromthe scope and spirit of the invention as set forth in the followingclaims.

For example, the steered wheels of the vehicle are not limited to frontwheels, and may be rear wheels or both the rear and front wheels.

The steering controller 20C, in accordance with an embodiment of theinvention, may be constituted such that the vehicle speed calculatingunit 29 is omitted, and the lock coefficient setting unit 28 sets a lockcoefficient K based on the difference Δα only.

According to the present invention, a steering actuator is eliminated,thereby to provide a steering control apparatus, a steering controlsystem and a steering control program in the SBW scheme, which enhanceflexibility of a steering-relevant design.

The embodiments according to the present invention have been explainedas aforementioned. However, the embodiments of the present invention arenot limited to those explanations, and those skilled in the artascertain the essential characteristics of the present invention and canmake the various modifications and variations to the present inventionto adapt it to various usages and conditions without departing from thespirit and scope of the claims.

1. A computer program product embodied on a computer readable storagemedium, the computer program product being encoded with instructionsconfigured to control a controller to perform a process for controllingthe steering of a steer-by-wire type vehicle, wherein the vehiclecomprises steered-wheel motors that apply separate torques to the rightand left steered wheels respectively, wherein a center of a contact facethereof and a kingpin point thereof are offset in a lateral direction ofthe vehicle, the process comprising: setting a target steering angle forthe vehicle based on a steering amount of the vehicle; setting a targettorque difference between the right and left steered wheels based on thetarget steering angle; setting a target driving/braking force for thevehicle based on an acceleration amount and a braking amount of thevehicle; setting each target torque for the right and left steeredwheels based on the target torque difference and the targetdriving/braking force; and controlling driving of the steered-wheelmotors based on the target torque, wherein the kingpin point ispositioned closer to the vehicle on an inside portion of an insidesidewall of a steered wheel.
 2. The computer program product accordingto claim 1, wherein the vehicle further comprises a lock mechanismconfigured to lock a rod member, wherein the process further comprisesdetermining that it is required to lock the rod member when a differencebetween a steering angle of the vehicle and the target steering angle isequal to a first predetermined value or smaller, and wherein the vehiclefurther comprises a lock mechanism driving unit configured to controldriving of the lock mechanism based on the determination that it isrequired to lock the rod member.
 3. The computer program productaccording to claim 1, wherein the vehicle further comprises a lockmechanism configured to lock a rod member and configured to continuouslyor intermittently change a locking force on the rod member, wherein theprocess further comprises setting a lock coefficient based on a steeringangle of the vehicle and the target steering angle, with reference to alock coefficient map in which at least the lock coefficient iscorrelated with a difference between a steering angle of the vehicle andthe target steering angle, and wherein the vehicle further comprises alock mechanism driving unit configured to control a driving of the lockmechanism to provide the locking force on the rod member based on thelock coefficient.
 4. A steering control system that controls steering ofa steer-by-wire type vehicle comprising steered-wheel motors that applyseparate torques to the right and left steered wheels respectively,wherein a center of a contact face thereof and a kingpin point thereofare offset in a lateral direction of the vehicle, the system comprising:a target steering angle setting unit configured to set a target steeringangle for the vehicle based on a steering amount of the vehicle; atarget torque difference setting unit configured to set a target torquedifference between the right and left steered wheels based on the targetsteering angle; a target driving/braking force setting unit configuredto set a target driving/braking force for the vehicle based on anacceleration amount and a braking amount of the vehicle; a target torquesetting unit configured to set each target torque for the right and leftsteered wheels based on the target torque difference and the targetdriving/braking force; and a motor driving unit for controlling drivingof the steered-wheel motors based on the target torque, wherein thekingpin point is positioned closer to the vehicle on an inside portionof an inside sidewall of a steered wheel.
 5. The steering control systemaccording to claim 4, wherein the vehicle further comprises a lockmechanism configured to lock a rod member, and wherein the steeringcontrol system further comprises a lock necessity determining unitconfigured to determine that it is necessary to lock the rod member whena difference between a steering angle of the vehicle and the targetsteering angle is equal to a first predetermined value or smaller, and alock driving mechanism unit configured to control driving of the lockmechanism based on a determination of the lock necessity determiningunit.
 6. The steering control system according to claim 4, wherein thevehicle further comprises a lock mechanism configured to lock a rodmember and continuously or intermittently change a locking force on therod member, and wherein the steering control system further comprises alock coefficient setting unit configured to store a lock coefficient mapfor at least correlating a lock coefficient with a difference between asteering angle of the vehicle and the target steering angle, and furtherconfigured to set the lock coefficient with reference to the lockcoefficient map based on the steering angle of the vehicle and thetarget steering angle, and a lock mechanism configured to controldriving of the lock mechanism to provide the locking force on the rodmember based on the lock coefficient.
 7. The steering control systemaccording to claim 6, wherein the lock coefficient map is configuredsuch that, the greater the difference between the steering angle of thevehicle and the target steering angle becomes, the smaller the lockcoefficient becomes, and the lock coefficient becomes constant when thedifference between the steering angle of the vehicle and the targetsteering angle is equal to a second predetermined value or greater. 8.The steering control system according to claim 6, wherein the lockcoefficient map further correlates a vehicle speed with the lockcoefficient, and is configured such that, the greater the vehicle speedbecomes, the greater the lock coefficient becomes.
 9. The steeringcontrol system according to claim 7, wherein the lock coefficient mapfurther correlates a vehicle speed with the lock coefficient, and isconfigured such that, the greater the vehicle speed becomes, the greaterthe lock coefficient becomes.